1. Technical Field
The present invention pertains to ammonia vapor generation systems and methods for delivering high purity ammonia to facilities at desired flow rates.
2. Discussion of the Art
High purity ammonia is required in ever increasing quantities for the manufacture of semiconductor devices as well as a number of manufacturing processes. In such manufacturing processes, ammonia is typically shipped in liquid form in containers to facilities for use in each process. Due to its low vapor pressure at normal ambient temperatures and the high flow rates that are typically required, the ammonia must be heated to achieve a suitable vapor pressure for delivering ammonia at a suitable flow rate.
In addition, ammonia gas experiences a significant Joules-Thompson cooling effect when forced through an orifice, such as a regulator. Thus, the temperature and pressure of the ammonia gas must be closely monitored to prevent undesirable temperature changes from occurring within the manufacturing process. Further, in order to obtain the purity required for the ammonia gas, many manufacturing processes require purification equipment disposed within the manufacturing system. Impurities that need to be removed from the ammonia feed include Group I and II metals, as well as amines, oxides and carbonates of these metals.
Thus, there exists a need to provide an ammonia vapor generation system that provides high purity ammonia gas at a suitable and relatively constant flow rate and a selected temperature and pressure.
Accordingly, it is an object of the present invention to provide ammonia gas at a selected flow rate for a particular process.
It is another object of the present invention to control the temperature and pressure of the ammonia gas as it is delivered to a particular process.
It is a further object of the present invention to provide ammonia gas in an efficient manner and at a suitable purity level for a particular process.
It is still another object of the present invention to provide an ammonia gas at a substantially continuous flow rate and a substantially constant pressure.
The aforesaid objects are achieved individually and/or in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
According to the present invention, an ammonia vapor generation system includes a liquid ammonia supply source, a vapor generation tank including an inlet to receive liquid ammonia from the supply source and an outlet to discharge ammonia gas from the vapor generation tank, a first heat transfer system to provide external cooling to the vapor generation tank, and a second heat transfer system to heat liquid ammonia within the vapor generation tank. The first and second heat transfer systems facilitate the discharge of ammonia gas from the vapor generation tank at a substantially constant flow rate. In particular, the first heat transfer system cools liquid ammonia being fed to the vapor generation tank and maintains the liquid ammonia within the tank at a selected temperature during a first time period after initiation of system operation. The second heat transfer system provides heat to the liquid ammonia within the tank after the first time period. The system is capable of providing a constant flow rate of ammonia gas from the vapor generation tank of at least about 1000 standard liters per minute.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components.